Such circuit configurations are employed for example in microdisplay applications for front and rear projection. In this situation it is necessary to turn on and turn off the current flowing through the LEDs and thus the light emitted by the LEDs in the μs range. In this situation, in projection systems which are not situated in the absolute high-end range it can be useful to drive the different channels, which correspond to different colors, successively in time, in other words sequentially. This means that the equipment costs can be considerably reduced because there is no need to provide one power source per channel, but one power source in total can be used for all channels. By preference, this power source is designed for very rapid switching operations in order to achieve the best possible light yield.
FIG. 1 shows a schematic illustration of a generic circuit configuration known from the prior art. In this situation, a linear regulator is used as the power source. This includes the operational amplifier OPAMPlin, a feedback resistor Rlin,FB arranged between the output of the operational amplifier OPAMPlin and its minus input, and a resistor Rlin,gate arranged coupled in series between the output of the operational amplifier OPAMPlin and a power transistor Tlin driven in linear fashion used for setting the current strength. The plus input of the operational amplifier OPAMPlin is coupled to a control voltage Uin which defines how high is the current to be released by the transistor Tlin, flowing through the respectively closed switch S1 to Sn and through the respectively assigned diode D1 to Dn. The switches S1 to Sn are driven in saturation and must be capable of switching the full current flowing through the LEDs. On the anode side, the diodes D1 to Dn can be coupled to a common supply voltage Usupply, while on the cathode side, a current sensing resistor Rshunt is coupled between the cathode of each diode D1 to Dn and a reference potential. The voltage drop at the current sensing resistor Rshunt is coupled to the minus input of the operational amplifier OPAMPlin.
The switches S1 to Sn are turned on sequentially by a control circuit which is not shown.
A disadvantage of the circuit configuration shown in FIG. 1 is the fact that it requires elaborate measures for heat dissipation.